Horizontal wells have been used extensively in heterogeneous reservoirs to intersect fractures and/or to reduce the detrimental effects of gas coning and water coning. It has been shown that such wells are capable of higher oil production rates than vertical wells drilled in the same reservoir. In most cases, the higher productivity more than offsets the higher cost of drilling and completion of the horizontal well. Theory predicts that the use of multiple horizontal drainholes correspondingly multiplies the total well productivity. Indeed many vertical cased wells connected to twin or multiple horizontal drainholes of medium (500-200 ft) and short (150-40 ft) radius of curvature have been successfully used in compact oil reservoirs, such as the Austin Chalk, in which open hole completion of the drainholes is applicable.
In many clastic reservoirs, however, the strength of unconsolidated sands or of friable sandstones may be insufficient to keep horizontal drainholes open. In such a case, the horizontal and deviated parts of each drainhole must be kept open with a tubular liner which is tied to the vertical casing using conventional equipment and known assembly procedures. This has been done in many different clastic reservoirs, containing light or heavy oil, for horizontal wells consisting of a single liner-equipped drainhole.
A patented U.S. Pat. No. 4,787,465 drilling and completion technique for multiple drainholes of ultra-short (ca. 10 ft) radius of curvature has also been used in such sandy reservoirs, but the liners of the short multiple drainholes are not tied-in to the vertical casing and their inner diameter and curvature radius are too small to allow the use of conventional logging and cleaning tools.
The present invention addresses the problem of drilling, cementation and tie-in by pressure-tight connections to a casing of twin or multiple drainholes of medium to short radius of curvature (typically 500 ft to 40 ft) equipped with liners of sufficient diameter to allow the passage of available well logging, perforating, cementing and cleaning tools, for subsequent well maintenance and repairs.
The next step is to provide the means to bring up the reservoir fluids and/or to inject fluids from the surface into the reservoir through the drainhole liners. Depending upon the mode of exploitation of the well and field conditions, a great variety of tubing completion assemblies may be used for these purposes. The simplest, which allows only commingled flow from or into all drainholes simultaneously, does not even requires any additional equipment if vertical flow is through the casing, but it provides minimum operational flexibility and no safety controls. For these reasons, additional equipment (at least a properly sized production tubing or a kill string for safety, for instance, and often a hanger or a packer) will be used in the field. The tubing completion assembly which provides the greatest operational flexibility and safety is that which provides a direct connection of each drainhole separately to a tubing, thus leaving the casing/tubing annulus available for other uses. This is the type of tubing completion assembly which is included in the present invention. It also provides the means of implementing in this type of heterogeneous reservoirs the heavy oil recovery process and the injected steam quality conservation process described respectively in U.S. Pat. No. 4,706,751 and U.S. Pat. No. 5,085,275 using some of the equipment described in U.S. Pat. No. 5,052,482. The present invention, however, does not preclude the use of the already known simpler completion designs, whenever they are sufficient for the application considered. Known elements of downhole equipment (valve nipple joints, safety joints, retrievable plugs, etc . . . ) may also be added, as needed, to the novel tubing completion assembly to perform specific additional tasks.
Some of the reservoirs under consideration, especially those containing heavy oil, require artificial lift to bring the production stream to the surface. The present invention includes equipment providing the means of pumping produced fluids and of injecting steam and/or other gases in such wells equipped with multiple drainholes completed with liners. Sand production being frequent in such reservoirs, the drainholes may be gravel packed or equipped with screens or subjected to known sand consolidation techniques.
The desired well and drainholes configuration may be obtained either with entirely new well or by re-entry into an existing vertical cased well, in which case the required equipment and procedures are somewhat different.
In all cases it is intended to obtain leak-proof connections between the drainhole liners and the vertical casing and between the drainhole liners and the tubings used either for production, injection and pumping. The desirability of a system which can be installed in as few steps as possible and which can easily be disassembled during future work-over operations has led to develop downhole equipment and procedures, which conform with proven oil field safety practices.
Due to the complex nature of oil reservoirs, especially those made-up of clastic rocks deposited in agitated water (Fluvio-Deltaic environment, turbidite currents or near shore sedimentation) or those resulting from collen transport (Dunes), the presence of various sediment heterogeneities and fractures, together with other reservoir engineering considerations regarding water/oil and gas/oil contacts locations, reservoir fluid pressure and solution GOR of the produced oil, will dictate various well and drainhole configurations.
Although the most frequently applicable is that of twin drainholes with their respective horizontal sections oriented at 180 degrees from each other, the equipment, tools and procedures which will be described are not restricted to that single configuration. It will become apparent to those skilled in the art that similar equipment and procedures may be adapted to all other multiple drainhole configurations without departing from the spirit of this invention.
Ranked in increasing degrees of complexity, the cases of drilling, tie-in and completion of new wells include:
1) side by side drainholes kicked-off from the bottom of a vertical cased well, using a twin whipstock.
2) side by side drainholes connected by intermediate liners to the bottom of a vertical well,
3) side by side drainholes obtained from a deviated cased well,
4) stacked drainholes kicked-off one above the other from a new vertical cased well. Two different tie-in methods and equipment types will be described, one using telescopic liner stubs and telescopic connector tubes to tie-in and complete the well, the other using intermediate cemented liners and articulate connector tubes.
5) use of a single pump for both drainholes, located above the kick-off points,
6) conveyance of low GOR production streams from each drainhole through a syphon to a single pump located near the base of an oil sump well below the kick-off points,
7) pumping of each drainhole with a pump located at or near the start of the horizontal segment,
8) simultaneous injection of steam and/or gases into one drainhole while producing oil and water from the other drainhole, as taught in in U.S. Pat. Nos. 4,706,751 and in application No. 512,317, now U.S. Pat. No. 5,085,275.
For re-entry into an existing vertical cased well, modified equipment and procedures will be described, corresponding to cases similar to cases 1, 2, 4, 6, 7 and 8 above.